1. Technical Field
The present disclosure relates to the field of low temperature refrigeration. Particularly, this disclosure relates to low-vibration cryogenic devices and methods of use.
2. Background
Cryo-coolers are devices designed to cool samples to cryogenic temperatures, so that a user can use or make measurements on a cold sample.
One class of cryo-coolers achieve cooling of a sample by dripping or venting liquid helium onto a cold finger sample mount plate, where the sample is in vacuum, or alternatively, in a chamber of cold helium gas which surrounds the sample mount. The action of boiling off liquid helium effectively cools the sample to temperatures near or below 4 degrees Kelvin (4 K). Helium gas is then vented off into the atmosphere. This type of cryo-cooler is called an open-cycle cryo-cooler because of the fact the system has to be constantly fed helium for continuous operation. Helium is supplied to these systems through a transfer line from a dewar that contains liquid helium, or where there is a dewar above the sample mount. These systems inherently have little or no vibrations induced on the sample, which would only be induced by the dripping helium, or the external surroundings, however, open-cycle cryo-coolers are hard to maintain, and depend on an external continuous supply of liquid helium, which is hard to distribute and handle and is becoming increasingly expensive as helium is a non-renewable resource only attainable from mining, or as bi-products of other manufacturing processes. Open-cycle cryo-coolers also require somewhat constant maintenance in the delivery of the helium supply to the cryostat which is monitored and controlled.
To overcome the deficiencies of open cycle cryo-coolers, a class of cryo-coolers referred to as cryo-refrigerators or closed-cycle cryo-coolers have been developed. These are thermo-mechanical devices which provide cooling to a cold finger through the pressure cycling of helium gas. These systems require only a single charge of helium gas, which is then pressure-cycled in an ongoing refrigeration cycle. Cryogenic temperatures can vary, but are typically defined to be <4K for a class of cryo-cooler which use circulating helium gas as the coolant. These systems operate on steady wall power, and will run continuously without maintenance for intervals on the order of ten-thousand hours. Closed-cycle cryo-coolers are thus much more reliable over longer periods of time than open-cycle cryo-coolers, do not require an expensive liquid helium supply, and can operate un-attended.
However, while these cryo-refrigerator systems solve many problems for the end user, they have deficiencies of their own for applications where the measurement or experiment or sample is very sensitive to external perturbations, vibrations and/or acoustic noise. These problems arise with the cryo-refrigerator systems due to the mechanical noise and vibrations created by the pressure and temperature cycling of the cold-head. The vibrations are created by the normal operation of the cold-head, which propagate to the sample through mechanical connections through the cold finger to the sample.
Thus, there exists a need for a closed cycle cryo-cooler configuration in which a sample of choice can be cooled to cryogenic temperatures, but without having a direct coupling to the vibrations of the cold-finger, for sensitive and cryogenic applications. These deficiencies are not known to have been overcome in the prior art.
A number of attempts have been made to isolate vibrations of a closed-cycle cryo-cooler from a test sample. One attempt outlined in U.S. Pat. No. 5,327,733 seems to have improved vibration isolation, however the size of the apparatus and specialization of the lab required to accept such an apparatus is relatively large. This apparatus requires separate support structures, which in their preferred embodiment comprises a stiff structure protruding from a ceiling of a laboratory to support the cryo-cooler expander unit, while a similar support structure is mounted on a laboratory table specially designed to dampen vibrations, and accept the lower portion of the structure, which supports the sample. The table is filled with granular material to further dampen vibrations. This entire structure and included support hardware would be extremely hard to move, and if a move did happen modification of a laboratory at the new location would be required. The described design also has multiple independent supports for the entire system.
U.S. Pat. No. 4,854,131 describes a vibration isolation system which has a support structure supported by the same flange that supports the cryo-cooler. This would allow vibrations to travel directly from the cold head to the supported sample. This patent also includes a Joule-Thomson cooler attached to the end of a conventional closed cycle cryo-cooler, which creates a more complex system than needed to achieve temperatures of 15 K or lower.
In U.S. Pat. No. 3,894,403 a vibration isolation system is described which is similar to U.S. Pat. No. 5,327,733 in its structure for supporting the sample. These two inventions use a convective gas such as helium to transfer the thermal energy from the sample to the cryo-cooler. The use of gas or liquid to transfer thermal energy is not feasible in certain situations such as remote installations due to supply problems, for example on a ship or submarine.
Another example of an attempt at vibration isolation of a sample is found in U.S. Pat. No. 5,129,232 where a sample is connected to the cold-finger with strap links. The sample is not supported except through the flexible thermal links. This system is missing a support structure designed to stabilize the sample.
Two U.S. Pat. Nos. 4,394,819 and 4,161,747 describe a sample directly coupled to a stable reference plane. One mounts the cryo-cooler in a floated position that allows movement of the cryo-cooler, while the second is solidly mounted. The floated mount style can have good vibration dampening, however, it is not robust for remote installations, especially when multiple isolator pads and bellows have to be maintained, and is therefore not suitable for applications requiring reliability, such as on a ship or submarine. Both systems use a support structure, however, U.S. Pat. No. 4,394,819 does not mention what the structure is, while U.S. Pat. No. 4,161,747 mentions Nylon rods with copper spacers to create a support structure.
Regarding the thermal links, U.S. Pat. No. 4,869,068 uses a single piece of folded, thermally conductive material to transfer heat while allowing motion. This disclosure is insufficient, however, as the stiffness is significantly higher in this geometry than otherwise possible by using multiple elements to transfer thermal energy.
Conventional thermal links are typically made from multiple elements of thermally conductive material, stacked to form a single thermal connection, as in U.S. Pat. No. 5,129,232. This type of thermal link provides some flexibility in three orthogonal directions, however, the stiffness is only minimized in one direction, and even this minimized stiffness is less flexible than otherwise possible.
Thermal links using wires oriented in a twisted orientations are disclosed in U.S. Pat. No. 5,317,879. These thermal links serve to provide similar flexibility in any three orthogonal directions, however, the twisted orientation of the wires cause binding and causes interaction between individual wires, making it less flexible. Thermal links that use braided wires exist, as in U.S. Pat. No. 4,854,131, however, are similarly disadvantageous as the wires bind on each other due to the braded wire orientation.
A thermal connection is also disclosed in U.S. Pat. No. 5,077,637, which utilizes wires in a housing. Disadvantageously, the housing material limits the flexibility of the thermal connection, making it undesirably stiff.